In the field of subsea hydrocarbon extraction, subsea wells often have two bores. This often consists of a main mainbore, often used for main, and an annulus which may also be used for gas injection. When intervening in a subsea well to undertake maintenance or abandonment work, it is often necessary to access both the main bore and the annulus. Often, previously when intervening in subsea wells using a subsea monobore riser or lubricator system access to only one of the bores is possible at a time.
When a subsea monobore riser or lubricator system is used, once access is gained to one of the bores and the work required on that bore is completed the monobore or lubricator system is recovered to the deck of the vessel. Should access also be required to the second bore a different adapter is fitted and tested, and the monobore riser or lubricator system is again lowered to the subsea well and the tools are inserted into the second bore to undertake the necessary work. The retrieval and deployment of a subsea riser or lubricator system is not a trivial exercise and each time the operation is performed the possibility of complications occurs.
The removal and replacement of the monobore riser or lubricator system including the various pressure testing steps is time consuming and therefore expensive. It is desired to find an alternative way of accessing both bores when using a subsea monobore riser or lubricator system.
One way of gaining access to both bores without the need to redeploy the riser or lubricator system is the use of a dual bore riser or lubricator system. There is a significant cost associated with the use of a dual bore riser system including capital and operating cost as a much larger vessel is required.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of admission that the prior art forms part of the common general knowledge in Australia.